High levels of integration of circuits has produced large chips that can perform many tasks. However, the tightly packed transistors generate heat, especially at higher operating frequencies. Hotspots on the chip can cause erratic circuit function or even permanent damage.
Temperature-monitoring circuits can be added to the integrated circuit (IC) chip. Such on-chip temperature monitor circuits may detect high temperatures and activate a power controller to power down some or all of the chip's circuits, or perhaps reduce the operating frequency. Once the temperature falls sufficiently, the circuits may be powered back up or resume operation.
Ideally such temperature monitoring circuits use the same process technology as the other circuits on the chip. However, some temperature monitoring circuits require a more expensive BiCMOS technology rather than standard complementary metal-oxide-semiconductor (CMOS).
FIG. 1 shows a simple prior art temperature monitoring circuit. Current source 10 drives a constant current I0 to diode 12, which sinks a current to produce an output voltage VO. The current through diode 12 is related to the base-emitter diode junction voltage, VBE, as:VBE=(kT/q)ln(IC/IS)
where k is Boltzman's constant, T is the absolute temperature, q is the elementary charge, IC is the collector current through diode 12, and IS is the saturation current. Unfortunately, the saturation current itself also varies with temperature, so two variables depend on temperature, T and IS.
The output voltage VO is equal to the base-emitter junction voltage, such as 0.6 volts, plus a temperature-sensitive term of about −2 mV per degree Kelvin, or −2 mV/K. This sensitivity to temperature is fixed by the manufacturing process.
A drawback of the simple circuit in FIG. 1 is that a constant current source 10 is needed. Linearity of the output voltage with temperature can be degraded due to non-ideal characteristics of real current sources. Also, the amount of temperature sensitivity is relatively small. The temperature sensitivity of the saturation current IS also introduces non-linearities to the temperature sensor.
FIG. 2 shows another prior art temperature sensing circuit. The temperature sensitivity can be scaled by the ratio of resistors 16, 14, allowing for greater temperature sensitivity. See U.S. Pat. No. 7,368,973 to Sato et al. A base current cancellation circuit includes transistors 22, 24, 26, while transistor 20 is used rather than a diode to generate VO. The output voltage VO=(0.6V−2 mV/K)*(1+R16/R14). Thus the temperature sensitivity is scaled by resistor ratio R16/R14.
Unfortunately, an ideal current source 10 is still needed, and a BiCMOS process is used. Linearity is degraded since the saturation current term cannot be cancelled. Also, the voltage headroom is reduced due to the number of stacked transistors. This reduced headroom can be a problem for reduced power-supply voltages.
What is desired is an improved temperature sensing circuit. A temperature sensing circuit that does not require BiCMOS but uses a standard CMOS process is desired. A temperature sensor that scales temperature sensitivity using a resistor ratio, a current-mirror ration, or other methods, but does not have a reduced voltage headroom is desirable. A temperature sensor that does not require an ideal current source, and than can cancel the saturation current term is also desirable.